Description: Returns a Base64 encoded string. This is
useful for being able to transfer binary data as ascii.

Subject Type: String

Arguments: No arguments

Return Type: String

Examples: We can Base64-Encoded an attribute named
"payload" by using the Expression ${payload:base64Encode()}
If the attribute payload had a value of "admin:admin" then the Expression
${payload:base64Encode()} will return
"YWRtaW46YWRtaW4=".